This invention relates to carbon fiber, and more particularly to extremely fine pitch-type carbon fiber and a preparation method of the same.
In general, pitch-type carbon fibers are broadly classified into two groups, that is, high performance carbon fiber (HP product) and general purpose carbon fiber (GP product).
High performance carbon fiber is prepared by spinning an optically anisotropic spinning pitch to give fiber in which liquid crystalline molecules are arranged parallel to the axial direction of the fiber, and subjecting the fiber to infusibilization and carbonization to form graphite crystals thereon. High performance carbon fiber having high strength and high modulus can thus be obtained.
General purpose carbon fiber, on the other hand, is prepared by spinning an optically isotropic pitch as it is, followed by carbonization. The resulting carbon fiber is a non-graphitic optically isotropic material. General purpose carbon fiber having moderate strength can thus be inexpensively obtained.
Studies are now being made on application of the above conventional carbon fibers to practical use, in such a field that their characteristics and properties are supposed to be utilizable.
Heretofore, the high performance carbon fiber has been mainly prepared by a melt spinning method, and the general purpose carbon fiber, a centrifugal spinning method. Carbon fibers prepared by either method have a diameter of approximately 8 to 15 .mu.m, and it is quite difficult to prepare fiber having a smaller diameter than the above by any conventionally known method.
Furthermore, carbon is intrinsically a brittle material, so that carbon fiber is inferior to fibers made from other materials in flexibility, and is easily broken.
For the above reasons, long carbon fiber requires careful handling; and short carbon fiber tends to be easily broken when preparing a composite material by mixing the carbon fiber with a plastic or concrete. In addition, papers, felts and mats which are produced by using short carbon fiber are poor in flexibility, so that they are easily damaged.
The above shortcomings can be eliminated if the diameter of the carbon fiber can be reduced. However, fine carbon fiber cannot be obtained by the conventional art due to mainly the below-described reasons:
In general, in the melt spinning method, a spinning pitch is discharged from a nozzle, and the discharged pitch fiber is wound up at high speed to finally obtain thin fiber. However, even the pitch fiber before being subjected to the winding has a low strength of approximately 0.4 kg/mm.sup.2, so that finally obtainable fine fiber is to have an extremely low strength. On the other hand, stretching force which is applied to pitch fiber during spinning is increased as the diameter of the fiber decreases, in other words, as the winding speed is increased. When the stretching force finally becomes higher than strength of the pitch fiber, the fiber snaps, and the spinning cannot be stably continued any more.
In the centrifugal spinning method, on the other hand, a spinning pitch is discharged from a nozzle which is revolving at high speed, and the discharged pitch fiber is blown off by centrifugal force, thereby obtaining thin fiber. However, thin fiber has a low mass, so that it cannot be easily applied with force of inertia. For this reason, fiber cannot be made thin without limitation.